For electronic components such as semiconductor devices, copper, copper alloy, 42 alloy, and the like are used for base materials for external terminals. Oxidation occurs on the surface of a terminal made of such a base metal. This might cause conduction failures due to soldering failures and the like. Therefore, in general, oxidation is prevented by forming a protective layer (plating layer) on the terminal surface by plating or the like.
In a case in which a tin alloy or a zinc alloy is used as a material for a plating layer, an alloy containing lead has been conventionally used. In recent years, in view of the reduction in environmental burdens, lead-free preparation has been required. As a plating layer material used for the above terminal, materials containing no lead, such as Sn, and Sn—Cu, Sn—Bi, and Sn—Ag alloys, have been used. However, when the surface of a terminal provided to an electronic component is subjected to plating processing with a lead-free material, whiskers, which are needle monocrystals of tin (Sn) or the like, are formed on a plating layer.
In recent years, for example, electronic components such as semiconductor devices having IC-chip-mounted lead frames are required to be further downsized. As a result, inter-terminal intervals have been narrowed down to approximately several hundred micrometers. Whiskers described above grow to a length of several hundreds of micrometers in some cases. Thus, in a case in which the inter-terminal interval is as narrow as several hundred micrometers as described above, an inter-terminal short circuit might be created as a result of whisker formation. Therefore, it is necessary to take measures to suppress whisker formation.
Many suggestions have been made regarding such measures. For instance, Patent Document 1 describes that whisker formation on a plating layer can be suppressed in the following manner. Upon formation of a lead-free Sn plating layer on the surface of a lead base material constituting an external terminal of an electronic component, the size of crystal particles constituting the plating layer is increased such that the grain boundary size per unit volume in the plating layer is decreased to become as small as possible.
In addition, Patent Document 2 describes that whisker formation on a plating layer can be suppressed in the following manner. Upon formation of a plating layer containing, as a main constituent, Sn on the external terminal surface, a low-melting-point alloy layer comprising Sn or an alloy of Sn and a different metal and a stress relief layer comprising a non-Sn single metal are alternately laminated to form a laminate having at least 3 layers.    [Patent Document 1] JP Patent Publication (Kokai) No. 2005-86158 A    [Patent Document 2] JP Patent Publication (Kokai) No. 2004-128399 A